1. Field of the Invention
The present invention relates generally to prepreg systems that utilize bismaleimide resins as a principal ingredient. More particularly, the present invention is directed to improving the storage stability, handling characteristics and curability of such prepreg systems.
2. Description of Related Art
Composite materials are used extensively in the aerospace industry and in other situations where high strength and light weight are desired. Composites typically include fibers and polymer resin as the two principal elements. A wide range of fiber types has been used in composites. Glass, graphite, carbon and ceramic fiber are common. The fibers can be chopped, randomly oriented, unidirectional in orientation or woven into fabric. The fibers used in composite materials have diameters that range from extremely small to relatively large. Although it is possible to make composites using large diameter fibers, the more common practice is to take thousands of fibers having extremely small diameters and form them into individual bundles known as tows. These multi-fiber tows are much stronger and more flexible than single fibers having the same overall diameter. The tows can be woven into fabric in the same manner a conventional yarns. Alternatively, the tows are arranged in parallel to provide a unidirectional fiber orientation or they can be randomly oriented.
Thermosetting resins have been widely used as the resin matrix in composite materials due to their high strength and high temperature capabilities. Bismaleimide resins have been a particularly popular thermosetting resins for aerospace applications due to their high glass transition temperature, their ability to withstand hot/wet environments and their low smoke and toxicant emissions during combustion. An initial problem with composites that utilized bismaleimide resins was that they were relatively brittle and had low damage tolerance. As a result, thermoplastic materials were incorporated into the bismaleimide resin matrix to provide a “toughened” resin. For examples of toughened bismaleimide resins, see U.S. Pat. Nos. 5,248,711 and 5,037,689.
There are a number of ways to combine the resin with the fibers to form the final composite material. One approach, which has been in use for years, is to manually impregnate the fibers with activated resin in-situ on a mold or other support structure. Heat is then used to cure resulting “lay-up”. This type of manual lay-up procedure is popular because it is simple and requires little, if any special tools. However, it is difficult to accurately control the amount of resin that is applied to the fibers and to insure that the resin is being uniformly impregnated into the fiber tows. In addition, the amounts of curing agent and other additives that are added to the resin may vary between lay-ups. As a result, manual impregnation methods are not typically used in aerospace applications where the combination of high strength and light weight is critical.
In order to avoid the above problems, it has been common practice to form a prefabricated lay-up (prepreg) that includes the fiber and resin matrix (resin, curing agents and any additives). The prepreg is made under manufacturing conditions that allow the amount and distribution of resin matrix within the prepreg to be carefully controlled. Once formed, the prepreg may be applied to a mold or other support surface in the same manner as a conventional hand lay-up. In general, prepregs are not used immediately after they are formed. Instead, they usually are stored for use at a later time.
There are a number of characteristics that are desirable in any prepreg. For example, the prepreg must be sufficiently flexible to allow application to the desired mold surface. In addition, the tackiness (or tack) of the prepreg must be such that the prepreg adheres to underlying prepreg layers in the mold while not being so sticky that it becomes disrupted when handled. The prepreg resin should remain stable during storage so that the handling characteristics (i.e. flexibility and tack) do not change. In addition, the resin should not flow away from the fibers or otherwise redistribute itself during storage. At the same time, the resin should have appropriate flow characteristics during cure to reduce porosity and provide good laminate quality.
Prepreg materials that utilize bismaleimide resins are prone to storage stability problems and loss of the tack necessary for prepreg handling and lay up of complex structures. In addition, upon cure, bismaleimide resin prepregs often suffer from excessive matrix flow. These flow problems are particularly troublesome when the prepreg is used in making a panel structure where prepreg face sheets are bonded to a honeycomb core. Accordingly, there is a continuing need to develop prepreg systems that provide all of the advantages of a toughened bismaleimide resin while at the same time providing the tack, flexibility, resin flow and storage characteristics that are required for a prepreg to be acceptable for use in making aerospace structures.